Diesel fuel composition

ABSTRACT

A diesel fuel composition which essentially comprises only paraffins, and which is characterised in that 1) the normal paraffins with 18 or fewer carbons constitute not less than 12% by mass, and 2) the proportion of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm is less than 6.5% in proton nuclear magnetic resonance ( 1 H-NMR) spectra, and which has an oxidation index OI of less than 1.10. OI is represented by the equation OI=0.247X−0.001Y−0.053, wherein X is the proportion (%) of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm in proton nuclear magnetic resonance ( 1 H-NMR) spectra; and Y is the content (mass %) of normal paraffins with 18 or fewer carbons.

The present invention relates to a diesel fuel composition. Morespecifically, it relates to a diesel fuel composition with superioroxidation stability while being constituted essentially only ofparaffins.

Various kinds of research have been undertaken in recent years to makeuse of Fischer-Tropsch fuels (referred to below as FT fuels) to fuelcars in Japan. What is meant by FT fuels are fuels obtained by synthesisfrom raw materials such as natural gas, coal or biomass, using theFischer-Tropsch process via synthesis gas, a mixture of carbon monoxideand hydrogen. They are often used under names corresponding to the rawmaterial. For example, for those where natural gas is the raw materialthe name GTL is often used, if coal is the raw material the name CTL maybe used, and if biomass is the raw material, the name BTL may be used.The term GTL is also sometimes used as a generic name for fuels obtainedby the Fischer-Tropsch process, but in the present invention the term FTfuels is used for fuels obtained by the Fischer-Tropsch process and FTfuels are deemed to include GTL, CTL and BTL.

As mentioned above, these FT fuels are expected to be used asalternatives to petroleum because they are synthesised from rawmaterials such as natural gas, coal and biomass and also, because theydo not contain sulphur or aromatic hydrocarbons, they are expected to beused as diesel fuels which are better for the environment, in that theylimit the emission of sulphur oxides and particulate matter (PM) fromengines. They have already been made available commercially in someareas, as reported for example in “The marketability of liquid fuelsfrom natural gas (GTL)”, [Energy Economics], November 2001 issue.

It is acknowledged that diesel fuels, when oxidised, change colour, formsludge and have increased viscosity, for example, and it is also knownthat peroxides formed by the oxidation cause deterioration of elements(rubbers and metals) in a vehicle's fuel system. For this reason,oxidation stability is an important indicator for evaluation of thequalitative stability of a diesel fuel, and it is desirable for a dieselfuel to have superior oxidation stability. In recent years, dieselengines have been equipped with common rail fuel injection systems as ameans of reducing particulates (or particulate matter, referred to belowas PM) in exhaust gases. The structure of these common rail fuelinjection systems is such that surplus fuel that has been conveyed underpressure to the injectors but that has not been injected into thecombustion chamber is returned to the fuel tank via a return pipe. Sincethis fuel returning to the fuel tank (the return fuel) is at an elevatedtemperature, oxidation of the diesel fuel within the fuel tank ispromoted, so that there is even more than hitherto a requirement toincrease the oxidation stability of diesel fuels, including FT fuels.

As regards the oxidation stability of diesel fuels, it is widely andgenerally known that oxidation stability can be improved by the additionof amine-based and phenol-based anti-oxidants of various kinds, andthere have been attempts to add oxidation stabilisers also to thefractions corresponding to the diesel oils of FT fuels (referred tobelow as FT diesel oils). As an example of this, mention may be made ofJP-A-2008-214369. There, it is disclosed that it is possible to increasethe oxidation stability at elevated temperatures by blending ananti-oxidant into a fraction corresponding to diesel oil for GTL fuels(GTL diesel fuel). However, the amount of anti-oxidant required in orderto obtain the desired effect becomes large for fuels that have pooroxidation stability and production costs are raised. In addition, as theamount of anti-oxidant increases, there are problems in that theanti-oxidant is prone to separate out at lower temperatures, orconversely, if the amount added is small, problems in that, after theeffect of the anti-oxidant has been used up during oxidation, theoxidation stability will deteriorate significantly and there may be adeleterious impact such as corrosion of metallic elements of thevehicle's fuel system.

JP-A-2008-266617 has, therefore, proposed maintaining the oxidationstability of diesel fuels without adding anti-oxidants. There, at leastone kind of polycyclic aromatic compound selected from the groupcomprised of anthracenes and dialkylnaphthalenes is blended with an FTdiesel oil to ensure oxidation stability.

However, it is known that polycyclic aromatic hydrocarbons in fuels areconnected with the polycyclic aromatic hydrocarbons in diesel exhaustgases which are acknowledged as carcinogenic in many substances, and itis more preferable if the content of polycyclic aromatic hydrocarbons infuels is as low as possible. Also, one of the features of FT diesel oilsis that they do not give rise to the environmental problems associatedwith aromatics and sulphur, and that feature is attributable to theirbeing formed only of paraffins. There have been problems in the priorart in dealing with the oxidation stability of FT diesel oils byblending in fuel compositions other than FT diesel oils, in that thisbasic feature of FT diesel oils is not sufficiently brought out.

The aim of the present invention, therefore, is to offer a diesel fuelcomposition with superior oxidation stability while being constitutedessentially only of paraffins.

The diesel fuel composition according to the present inventionessentially comprises only paraffins, and is characterised in that

1) the normal paraffins with 18 or fewer carbons constitute not lessthan 12% by mass, and2) the proportion of the total peak area of the peak group at chemicalshifts of 1.45 to 2.25 ppm relative to the total peak area of the peakgroup at chemical shifts of 1.00 to 1.45 ppm is less than 6.5% in protonnuclear magnetic resonance (¹H-NMR) spectra,and satisfies the condition that the oxidation index OI represented bythe following equation is less than 1.10:

OI=0.247X−0.001Y−0.053

wherein X is the proportion (%) of the total peak area of the peak groupat chemical shifts of 1.45 to 2.25 ppm relative to the total peak areaof the peak group at chemical shifts of 1.00 to 1.45 ppm in protonnuclear magnetic resonance (¹H-NMR) spectra; and Y is the content (mass%) of normal paraffins with 18 or fewer carbons.

It is undesirable here if the normal paraffins with 18 or fewer carbonsare less than 12% by mass, if the proportion of the total peak area ofthe peak group at chemical shifts of 1.45 to 2.25 ppm relative to thetotal peak area of the peak group at chemical shifts of 1.00 to 1.45 ppmis 6.5% or more in proton nuclear magnetic resonance (¹H-NMR) spectra,or if the oxidation index OI is 1.10 or more, because then the oxidationstability is impaired. To increase the oxidation stability even further,it is preferable if the oxidation index OI is less than 0.9, and morepreferably less than 0.7.

The diesel fuel composition relating to the present invention may alsobe such that the increase in the total acid number between before andafter an oxidation stability test is not more than 1.3 mg KOH/g. Thepreferred increase in the total acid number between before and after anoxidation stability test is 1.0 mg KOH/g, but more preferably 0.9 mgKOH/g. What is meant by an oxidation stability test in the presentinvention is an oxidation test performed as in ASTM D2274 underconditions of oxygen bubbling for 16 hours but with the test temperaturechanged to 115° C.

In addition, what is meant in the present invention by being essentiallycomposed only of paraffins is that the main constituent does not containstyrene compounds or diene compounds, or condensed polycyclic aromatics.Containing compositions of other than paraffins as impurities istolerated. For example, an FT diesel oil in which the total mass orvolume of isoparaffins and normal paraffins is not less than 99% of thewhole, excluding tiny impurities, is a diesel fuel compositionconstituted essentially only of paraffins suitable for the presentinvention. Also, it is possible to add suitable additives within a rangethat does not go beyond the scope of the present invention, for examplea range that does not cause the problems of cost or separation in theprior art.

As additives, mention may be made of lubricity improvers to prevent wearof, for example, fuel feed-pump parts. It is also possible to use, asthe lubricity improvers, any known lubricity improvers provided they aremiscible in paraffins. Typical lubricity improvers are commercialacid-based lubricity improvers which have fatty acids as their mainconstituent, and ester-based lubricity improvers which have as theirmain constituent glycerin mono fatty acid esters. These compounds may beused singly or in combinations of two or more kinds. The fatty acidsused in these lubricity improvers are preferably those that have astheir main constituent a mixture of unsaturated fatty acids ofapproximately 12 to 22 carbons, but preferably about 18 carbons, that isoleic acid, linolic acid and linolenic acid. The lubricity improver maybe added so that the wear scar WS 1.4 value in an HFRR (High FrequencyReciprocating Rig) of the fuel composition after addition of thelubricity improver is not more than 500 μm, but preferably not more than460 μm, and the amount thereof is usually 50 to 1000 ppm. The WS 1.4value in an HFRR here refers to the value obtained in accordance withthe Japanese Petroleum Institute standard JPI-5S-50-98 “Gas oil—Methodfor testing lubricity”.

Furthermore, as examples of other additives, mention may be made ofdetergents such as amine salts of alkenyl succinate derivatives, metaldeactivators such as salicylidene derivatives, de-icing agents such aspolyglycol ethers, rust inhibitors such as aliphatic amines and alkenylsuccinate esters, anti-static agents such as anionic and cationicamphoteric surfactants, and silicone-based defoaming agents. Theseadditives may be used singly or in combinations of two or more kinds.The amount added may be selected according to use, but will be, forexample, not more than 0.2% by mass relative to the fuel oilcomposition.

According to the present invention, it is possible to obtain a dieselfuel composition with excellent oxidation stability although comprisedessentially only of paraffins. It having been inferred that theproportion of branches relative to the length of linear molecules isconnected with the oxidation stability, and various trials having beencarried out, it has been discovered that there was a correlation therewith the chemical shifts in proton nuclear magnetic resonance (¹H-NMR)spectra. In addition, whilst it is generally believed that oxidationstability is subject to an influence that depends on the molecularweight and that if the molecular weight is large, this has a favourableimpact on oxidation stability, it has been discovered that if the normalparaffins with 18 or fewer carbons are not less than a specifiedproportion, this imparts an improvement in oxidation stability. Thepresent invention is based on these novel findings.

Low-temperature flow characteristics are required of diesel fuelcompositions, to take account of use during winter or in cold regions.From the standpoint of improving these low-temperature flowcharacteristics, it is generally preferable if there are moreiso-paraffins. On the other hand, from the standpoint of oxidationstability it is preferable to have more normal paraffins. In otherwords, low-temperature flow characteristics and oxidation stability showopposing behaviours in the composition. However, because it is possibleto improve low-temperature flow characteristics by means of additives,it is possible to achieve improvement in oxidation stability whiletaking low-temperature flow characteristics into account by suitable useof additives.

For low-temperature fluidity improvers it is possible to use any knownlow-temperature fluidity improvers provided they are miscible withparaffins. Typical low-temperature fluidity improvers are commerciallow-temperature fluidity improvers such as ethylene-vinyl acetatecopolymers, ethylene-alkylacrylate copolymers, alkenyl succinamides,chlorinated polyethylenes, or polyalkyl acrylates. These compounds maybe used singly or in combinations of two or more kinds. Of these,ethylene-vinyl acetate copolymers and alkenyl succinamides areespecially preferred. As to the amount of the low temperature fluidityimprover, for example a suitable amount may be blended in so as tosatisfy the pour points and cold filter plugging points specified in JISK 2204, which is the JIS standard for diesel fuel, but normally theamount will be 50 to 1000 ppm. The pour point here refers to the pourpoint obtained in accordance with JIS K 2269 “Testing methods for pourpoint and cloud point of crude oil and petroleum products”, and the coldfilter plugging point refers to the cold filter plugging point obtainedby JIS K 2288 “Petroleum products—Diesel fuel—Determination of coldfilter plugging point”.

Examples of the diesel fuel composition according to the presentinvention are explained here. However, the present invention is notlimited by the examples of given below.

The following base materials were used to produce diesel fuelcompositions comprising hydrocarbon fuel oils and FT fuels comprisedonly of polykerosene (paraffin) base materials, being oil mixtures withadjusted distillation characteristics and compositions. Table 1 showsthe characteristics of the base materials, while Tables 2 and 3 show thecharacteristics and compositions of the diesel fuel compositionsobtained. Table 3 also shows an FT fuel of the prior art as a referenceexample.

Base material A, base material B (synthetic paraffins): Taking as theraw materials by-product gases (butane and butylene fractions) whichhave as their main constituent light hydrocarbons with a carbon numberof 4 obtained, for example, from fluid catalytic cracking apparatus andthermal cracking apparatus during petroleum refining, an oligomerisationtreatment was carried out by means of the IFP/Axens Polynaphtha process,and after conversion selectively to hydrocarbon fractions of 10 to 24carbons, paraffin base materials with differing distillationcharacteristics and compositions were obtained via desulphurisation,hydrogenation treatment of olefins and the distillation process.

Base material C, base material D, base material E, base material F, basematerial G (FT fuels): natural gas was partially oxidised by means ofthe SMDS (Shell Middle Distillate Synthesis) process, and aftersynthesising the syngas derived from carbon monoxide and hydrogen(CO+H₂) to a waxy straight-chain alkyl hydrocarbon by means of aFischer-Tropsch reaction, hydrocracking and isomerisation were carriedout over a catalyst, and base materials being mixtures of normalparaffins and isoparaffins with differing distillation characteristicsand compositions were obtained.

TABLE 1 Units A B C D E F G Density g/cm³ 0.769 0.783 0.750 0.774 0.7770.785 0.757 @ 15° C. Distillation characteristics IBP ° C. 180.5 186.0189.5 258.5 204.0 208.5 155.0 10% ° C. 188.5 198.0 199.0 264.5 226.0244.0 172.0 50% ° C. 189.5 208.5 207.0 270.5 261.5 295.0 203.0 90% ° C.192.5 238.0 221.5 285.5 305.0 341.0 295.0 FBP ° C. 198.5 247.5 233.5294.5 315.5 358.0 312.0 Cetane 57.9 59.0 79.4 94.8 83.7 89.9 66.6 indexSulphur Mass ≦1 ≦1 ≦1 ≦1 ≦1 ≦1 ≦1 component ppm Paraffin Vol. % ≧99 ≧99≧99 ≧99 ≧99 ≧99 ≧99 component Aromatic Vol. % ≦1 ≦1 ≦1 ≦1 ≦1 ≦1 ≦1component

TABLE 2 Units Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 A Vol. % 50 B Vol. % CVol. % 50 100 D Vol. % 100 E Vol. % 100 F Vol. % 100 G Vol. % 100Density @ 15° C. g/cm³ 0.759 0.750 0.774 0.777 0.785 0.757 Distillationcharacteristics IBP ° C. 184.0 189.5 258.5 204.0 208.5 155.0 10% ° C.193.0 199.0 264.5 226.0 244.0 172.0 50% ° C. 197.5 207.0 270.5 261.5295.0 203.0 90% ° C. 212.0 221.5 285.5 305.0 341.0 295.0 FBP ° C. 228.5233.5 294.5 315.5 358.0 312.0 Cetane index 68.3 79.4 94.8 83.7 89.9 66.6Sulphur Mass ppm ≦1 ≦1 ≦1 ≦1 ≦1 ≦1 component Paraffin Vol. % ≧99 ≧99 ≧99≧99 ≧99 ≧99 component Aromatic Vol. % ≦1 ≦1 ≦1 ≦1 ≦1 ≦1 componentn-paraffins of Mass % 47.0 94.0 93.8 17.9 12.1 30.3 18 or fewer carbons

TABLE 3 Comparative Reference Units Example Example A Vol. % — B Vol. %85 — C Vol. % — D Vol. % 15 — E Vol. % — F Vol. % — G Vol. % — Density @15° C. g/cm³ 0.781 0.786 Distillation characteristics IBP ° C. 189.5209.0 10% ° C. 199.5 240.0 50% ° C. 212.0 290.0 90% ° C. 252.0 341.0 FBP° C. 274.5 355.0 Cetane index 61.5 85.9 Sulphur Mass ppm ≦1 ≦1 componentParaffin Vol. % ≧99 ≧99 component Aromatic Vol. % ≦1 ≦1 componentn-paraffins of Mass % 14.1 10.9 18 or fewer carbons

The various characteristics shown in Tables 1 and 2 were measured by themethods below.

Density @ 15° C.

Density at 15° C. measured in accordance with JIS K 2249 “Crude oil andpetroleum products—Determination of density and density/mass/volumeconversion tables”.

Distillation Characteristics

Distillation characteristics obtained in accordance with JIS K 2254“Petroleum products—Distillation test methods”.

Cetane Index

Refers to the cetane index measured in accordance with JIS K 2280“Petroleum products—Fuel oils—Determination of octane number and cetanenumber, and method for calculation of cetane index, 8. Method ofcalculating cetane index using the four-variable equation”. However,reference values were recorded in the case of FT fuels because they lieoutside the recommended appropriate scope of the cetane indexcalculation.

Sulphur Component

Sulphur content obtained in accordance with JIS K 2541-2 “Crudepetroleum and petroleum products—Determination of sulphur content, Part2: The microcoulometric titration-type oxidation method”.

Paraffin Component

Paraffin component measured in accordance with JPI-5S-49-97 “Petroleumproducts—Determination of hydrocarbon types—High performance liquidchromatography”.

Aromatic Component

Sum of monocyclic aromatic and dicyclic aromatic and tri- or highercyclic aromatic hydrocarbon components measured in accordance withJPI-5S-49-97 “Petroleum products—Determination of hydrocarbon types—Highperformance liquid chromatography”.

Proportion of n-Paraffins

For the content of normal paraffins with 18 or fewer carbons, gaschromatography in accordance with ASTM D 2887 “Standard test method forboiling point range distribution of petroleum fractions by gaschromatography” was used, and the normal paraffin content was obtainedfrom the peak area values of different carbon numbers from thechromatograms thus obtained.

The type of column in the gas chromatography was HP5 (length: 30 m,inside diameter: 0.32 mm, liquid layer thickness: 0.25 μm), and theanalysis conditions were as follows.

Column tank temperature rise condition: 35° C. (5 minutes)→10° C./minute(temperature rise) 320° C. (11.5 minutes)

Specimen volatilisation chamber conditions: 320° C. fixed, split ratio150:1

Detector part: 320° C.

In the case of Examples 1 to 6, the Comparative Example and theReference Example, proton nuclear magnetic resonance (¹H-NMR) spectraanalysis was carried out, and the proportion of the total peak area ofthe peak group at chemical shifts of 1.45 to 2.25 ppm relative to thetotal peak area of the peak group at chemical shifts of 1.00 to 1.45 ppmwas obtained. In addition, the oxidation index OI was obtained from theproportion of the total peak area obtained and the proportion of normalparaffins of carbon number 18 or less.

The results are shown in Tables 4 and 5.

As regards the oxidation stability test of Examples 1 to 6, theComparative Example and the Reference Example, the increment in thetotal acid number between before and after the acceleration test wasmeasured (referred to below as the acid number). The oxidation stabilitytest was performed in accordance with ASTM D2274 at 115° C. underconditions of oxygen bubbling for 16 hours. These results, too, areshown in Tables 4 and 5. The total acid number was measured inaccordance with JIS K 2501 “Petroleum products andlubricants—Determination of neutralisation value”.

TABLE 4 Units Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Proportion of % 1.40.6 0.7 3.0 2.9 2.8 ¹H-NMR peak area Oxidation 0.25 0.00 0.03 0.67 0.650.61 index OI Δ Acid mg KOH/g 0.17 0.07 0.06 0.50 0.85 0.37 number

TABLE 5 Comparative Reference Units Example Example Proportion of % 6.54.7 ¹H-NMR peak area Oxidation 1.54 1.10 index OI Δ Acid mg KOH/g 1.511.36 number

As shown in Table 4, although all Examples 1 to 6 were constituted onlyof paraffins, their Δ acid number was smaller than the ReferenceExample, which was an FT fuel of the prior art, and it was confirmedthat the oxidation stability was superior. The A acid number of theComparative Example was above the Reference Example and it may beconcluded that the oxidation stability was not improved.

It was confirmed that all the Examples of the present invention werecharacterised in that

1) the normal paraffins with 18 or fewer carbons constituted not lessthan 12% by mass, and2) the proportion of the total peak area of the peak group at chemicalshifts of 1.45 to 2.25 ppm relative to the total peak area of the peakgroup at chemical shifts of 1.00 to 1.45 ppm was less than 6.5% inproton nuclear magnetic resonance (¹H-NMR) spectra, and the oxidationindex OI represented by the following equation was less than 1.10.

OI=0.247X−0.001Y−0.053

wherein X is the proportion (%) of the total peak area of the peak groupat chemical shifts of 1.45 to 2.25 ppm relative to the total peak areaof the peak group at chemical shifts of 1.00 to 1.45 ppm in protonnuclear magnetic resonance (¹H-NMR) spectra; and Y is the content (mass%) of normal paraffins with 18 or fewer carbons).

1. A diesel fuel composition comprising essentially of paraffins, andwherein 1) the normal paraffins with 18 or fewer carbons constitute notless than 12% by mass, and 2) the proportion of the total peak area ofthe peak group at chemical shifts of 1.45 to 2.25 ppm relative to thetotal peak area of the peak group at chemical shifts of 1.00 to 1.45 ppmis less than 6.5% in proton nuclear magnetic resonance (¹H-NMR) spectra,and which has an oxidation index OI represented by the followingequation of less than 1.10:OI=0.247X−0.001Y−0.053 wherein X is the proportion (%) of the total peakarea of the peak group at chemical shifts of 1.45 to 2.25 ppm relativeto the total peak area of the peak group at chemical shifts of 1.00 to1.45 ppm in proton nuclear magnetic resonance (¹H-NMR) spectra; and Y isthe content (mass %) of normal paraffins with 18 or fewer carbons. 2.The diesel fuel composition of claim 1 wherein the increase in the totalacid number between before and after an oxidation stability test is notmore than 1.3 mg KOH/g.
 3. The diesel fuel composition of claim 2wherein the increase in the total acid number between before and afteran oxidation stability test is not more than 1.0 mg KOH/g.
 4. The dieselfuel composition of claim 3 wherein the increase in the total acidnumber between before and after an oxidation stability test is not morethan 0.9 mg KOH/g.
 5. The diesel fuel composition of claim 2 wherein theoxidation stability test is performed as in ASTM D2274 under conditionsof oxygen bubbling for 16 hours at a test temperature of 115° C.
 6. TheA diesel fuel composition of claim 1, which has an oxidation index OI ofless than 0.9.
 7. The diesel fuel composition of claim 6 which has anoxidation index OI of less than 0.7.
 8. The diesel fuel composition ofclaim 1, wherein the main constituent thereof does not contain styrenecompounds or diene compounds, or condensed polycyclic aromatics.
 9. Thediesel fuel composition of claim 1, wherein the total mass or volume ofisoparaffins and normal paraffins is not less than 99% of thecomposition.